doc: add a new doc/ directory, and move two markdown docs into them

I figure sooneror later we'll have more of these docs, hence let's give
them a clean place to be.

This leaves NEWS and README/README.md as well as the LICENSE texts in
the root directory of the project since that appears to be customary for
Free Software projects.

1 files changed, 442 insertions, 0 deletions

diff --git a/doc/CODING_STYLE b/doc/CODING_STYLE
new file mode 100644
index 0000000000..ae818126cb
--- /dev/null
+++ b/doc/CODING_STYLE
@@ -0,0 +1,442 @@
+- 8ch indent, no tabs, except for files in man/ which are 2ch indent,
+  and still no tabs
+
+- We prefer /* comments */ over // comments in code you commit, please. This
+  way // comments are left for developers to use for local, temporary
+  commenting of code for debug purposes (i.e. uncommittable stuff), making such
+  comments easily discernable from explanatory, documenting code comments
+  (i.e. committable stuff).
+
+- Don't break code lines too eagerly. We do *not* force line breaks at
+  80ch, all of today's screens should be much larger than that. But
+  then again, don't overdo it, ~119ch should be enough really.
+
+- Variables and functions *must* be static, unless they have a
+  prototype, and are supposed to be exported.
+
+- structs in MixedCase (with exceptions, such as public API structs),
+  variables + functions in lower_case.
+
+- The destructors always unregister the object from the next bigger
+  object, not the other way around
+
+- To minimize strict aliasing violations, we prefer unions over casting
+
+- For robustness reasons, destructors should be able to destruct
+  half-initialized objects, too
+
+- Error codes are returned as negative Exxx. e.g. return -EINVAL. There
+  are some exceptions: for constructors, it is OK to return NULL on
+  OOM. For lookup functions, NULL is fine too for "not found".
+
+  Be strict with this. When you write a function that can fail due to
+  more than one cause, it *really* should have "int" as return value
+  for the error code.
+
+- Do not bother with error checking whether writing to stdout/stderr
+  worked.
+
+- Do not log errors from "library" code, only do so from "main
+  program" code. (With one exception: it is OK to log with DEBUG level
+  from any code, with the exception of maybe inner loops).
+
+- Always check OOM. There is no excuse. In program code, you can use
+  "log_oom()" for then printing a short message, but not in "library" code.
+
+- Do not issue NSS requests (that includes user name and host name
+  lookups) from PID 1 as this might trigger deadlocks when those
+  lookups involve synchronously talking to services that we would need
+  to start up
+
+- Do not synchronously talk to any other service from PID 1, due to
+  risk of deadlocks
+
+- Avoid fixed-size string buffers, unless you really know the maximum
+  size and that maximum size is small. They are a source of errors,
+  since they possibly result in truncated strings. It is often nicer
+  to use dynamic memory, alloca() or VLAs. If you do allocate fixed-size
+  strings on the stack, then it is probably only OK if you either
+  use a maximum size such as LINE_MAX, or count in detail the maximum
+  size a string can have. (DECIMAL_STR_MAX and DECIMAL_STR_WIDTH
+  macros are your friends for this!)
+
+  Or in other words, if you use "char buf[256]" then you are likely
+  doing something wrong!
+
+- Stay uniform. For example, always use "usec_t" for time
+  values. Do not mix usec and msec, and usec and whatnot.
+
+- Make use of _cleanup_free_ and friends. It makes your code much
+  nicer to read!
+
+- Be exceptionally careful when formatting and parsing floating point
+  numbers. Their syntax is locale dependent (i.e. "5.000" in en_US is
+  generally understood as 5, while on de_DE as 5000.).
+
+- Try to use this:
+
+      void foo() {
+      }
+
+  instead of this:
+
+      void foo()
+      {
+      }
+
+  But it is OK if you do not.
+
+- Single-line "if" blocks should not be enclosed in {}. Use this:
+
+  if (foobar)
+          waldo();
+
+  instead of this:
+
+  if (foobar) {
+          waldo();
+  }
+
+- Do not write "foo ()", write "foo()".
+
+- Please use streq() and strneq() instead of strcmp(), strncmp() where applicable.
+
+- Please do not allocate variables on the stack in the middle of code,
+  even if C99 allows it. Wrong:
+
+  {
+          a = 5;
+          int b;
+          b = a;
+  }
+
+  Right:
+
+  {
+          int b;
+          a = 5;
+          b = a;
+  }
+
+- Unless you allocate an array, "double" is always the better choice
+  than "float". Processors speak "double" natively anyway, so this is
+  no speed benefit, and on calls like printf() "float"s get promoted
+  to "double"s anyway, so there is no point.
+
+- Do not mix function invocations with variable definitions in one
+  line. Wrong:
+
+  {
+          int a = foobar();
+          uint64_t x = 7;
+  }
+
+  Right:
+
+  {
+          int a;
+          uint64_t x = 7;
+
+          a = foobar();
+  }
+
+- Use "goto" for cleaning up, and only use it for that. i.e. you may
+  only jump to the end of a function, and little else. Never jump
+  backwards!
+
+- Think about the types you use. If a value cannot sensibly be
+  negative, do not use "int", but use "unsigned".
+
+- Use "char" only for actual characters. Use "uint8_t" or "int8_t"
+  when you actually mean a byte-sized signed or unsigned
+  integers. When referring to a generic byte, we generally prefer the
+  unsigned variant "uint8_t". Do not use types based on "short". They
+  *never* make sense. Use ints, longs, long longs, all in
+  unsigned+signed fashion, and the fixed size types
+  uint8_t/uint16_t/uint32_t/uint64_t/int8_t/int16_t/int32_t and so on,
+  as well as size_t, but nothing else. Do not use kernel types like
+  u32 and so on, leave that to the kernel.
+
+- Public API calls (i.e. functions exported by our shared libraries)
+  must be marked "_public_" and need to be prefixed with "sd_". No
+  other functions should be prefixed like that.
+
+- In public API calls, you *must* validate all your input arguments for
+  programming error with assert_return() and return a sensible return
+  code. In all other calls, it is recommended to check for programming
+  errors with a more brutal assert(). We are more forgiving to public
+  users than for ourselves! Note that assert() and assert_return()
+  really only should be used for detecting programming errors, not for
+  runtime errors. assert() and assert_return() by usage of _likely_()
+  inform the compiler that he should not expect these checks to fail,
+  and they inform fellow programmers about the expected validity and
+  range of parameters.
+
+- Never use strtol(), atoi() and similar calls. Use safe_atoli(),
+  safe_atou32() and suchlike instead. They are much nicer to use in
+  most cases and correctly check for parsing errors.
+
+- For every function you add, think about whether it is a "logging"
+  function or a "non-logging" function. "Logging" functions do logging
+  on their own, "non-logging" function never log on their own and
+  expect their callers to log. All functions in "library" code,
+  i.e. in src/shared/ and suchlike must be "non-logging". Every time a
+  "logging" function calls a "non-logging" function, it should log
+  about the resulting errors. If a "logging" function calls another
+  "logging" function, then it should not generate log messages, so
+  that log messages are not generated twice for the same errors.
+
+- Avoid static variables, except for caches and very few other
+  cases. Think about thread-safety! While most of our code is never
+  used in threaded environments, at least the library code should make
+  sure it works correctly in them. Instead of doing a lot of locking
+  for that, we tend to prefer using TLS to do per-thread caching (which
+  only works for small, fixed-size cache objects), or we disable
+  caching for any thread that is not the main thread. Use
+  is_main_thread() to detect whether the calling thread is the main
+  thread.
+
+- Command line option parsing:
+  - Do not print full help() on error, be specific about the error.
+  - Do not print messages to stdout on error.
+  - Do not POSIX_ME_HARDER unless necessary, i.e. avoid "+" in option string.
+
+- Do not write functions that clobber call-by-reference variables on
+  failure. Use temporary variables for these cases and change the
+  passed in variables only on success.
+
+- When you allocate a file descriptor, it should be made O_CLOEXEC
+  right from the beginning, as none of our files should leak to forked
+  binaries by default. Hence, whenever you open a file, O_CLOEXEC must
+  be specified, right from the beginning. This also applies to
+  sockets. Effectively this means that all invocations to:
+
+  a) open() must get O_CLOEXEC passed
+  b) socket() and socketpair() must get SOCK_CLOEXEC passed
+  c) recvmsg() must get MSG_CMSG_CLOEXEC set
+  d) F_DUPFD_CLOEXEC should be used instead of F_DUPFD, and so on
+  f) invocations of fopen() should take "e"
+
+- We never use the POSIX version of basename() (which glibc defines it in
+  libgen.h), only the GNU version (which glibc defines in string.h).
+  The only reason to include libgen.h is because dirname()
+  is needed. Every time you need that please immediately undefine
+  basename(), and add a comment about it, so that no code ever ends up
+  using the POSIX version!
+
+- Use the bool type for booleans, not integers. One exception: in public
+  headers (i.e those in src/systemd/sd-*.h) use integers after all, as "bool"
+  is C99 and in our public APIs we try to stick to C89 (with a few extension).
+
+- When you invoke certain calls like unlink(), or mkdir_p() and you
+  know it is safe to ignore the error it might return (because a later
+  call would detect the failure anyway, or because the error is in an
+  error path and you thus couldn't do anything about it anyway), then
+  make this clear by casting the invocation explicitly to (void). Code
+  checks like Coverity understand that, and will not complain about
+  ignored error codes. Hence, please use this:
+
+      (void) unlink("/foo/bar/baz");
+
+  instead of just this:
+
+      unlink("/foo/bar/baz");
+
+  Don't cast function calls to (void) that return no error
+  conditions. Specifically, the various xyz_unref() calls that return a NULL
+  object shouldn't be cast to (void), since not using the return value does not
+  hide any errors.
+
+- Don't invoke exit(), ever. It is not replacement for proper error
+  handling. Please escalate errors up your call chain, and use normal
+  "return" to exit from the main function of a process. If you
+  fork()ed off a child process, please use _exit() instead of exit(),
+  so that the exit handlers are not run.
+
+- Please never use dup(). Use fcntl(fd, F_DUPFD_CLOEXEC, 3)
+  instead. For two reason: first, you want O_CLOEXEC set on the new fd
+  (see above). Second, dup() will happily duplicate your fd as 0, 1,
+  2, i.e. stdin, stdout, stderr, should those fds be closed. Given the
+  special semantics of those fds, it's probably a good idea to avoid
+  them. F_DUPFD_CLOEXEC with "3" as parameter avoids them.
+
+- When you define a destructor or unref() call for an object, please
+  accept a NULL object and simply treat this as NOP. This is similar
+  to how libc free() works, which accepts NULL pointers and becomes a
+  NOP for them. By following this scheme a lot of if checks can be
+  removed before invoking your destructor, which makes the code
+  substantially more readable and robust.
+
+- Related to this: when you define a destructor or unref() call for an
+  object, please make it return the same type it takes and always
+  return NULL from it. This allows writing code like this:
+
+            p = foobar_unref(p);
+
+  which will always work regardless if p is initialized or not, and
+  guarantees that p is NULL afterwards, all in just one line.
+
+- Use alloca(), but never forget that it is not OK to invoke alloca()
+  within a loop or within function call parameters. alloca() memory is
+  released at the end of a function, and not at the end of a {}
+  block. Thus, if you invoke it in a loop, you keep increasing the
+  stack pointer without ever releasing memory again. (VLAs have better
+  behaviour in this case, so consider using them as an alternative.)
+  Regarding not using alloca() within function parameters, see the
+  BUGS section of the alloca(3) man page.
+
+- Use memzero() or even better zero() instead of memset(..., 0, ...)
+
+- Instead of using memzero()/memset() to initialize structs allocated
+  on the stack, please try to use c99 structure initializers. It's
+  short, prettier and actually even faster at execution. Hence:
+
+          struct foobar t = {
+                  .foo = 7,
+                  .bar = "bazz",
+          };
+
+  instead of:
+
+          struct foobar t;
+          zero(t);
+          t.foo = 7;
+          t.bar = "bazz";
+
+- When returning a return code from main(), please preferably use
+  EXIT_FAILURE and EXIT_SUCCESS as defined by libc.
+
+- The order in which header files are included doesn't matter too
+  much. systemd-internal headers must not rely on an include order, so
+  it is safe to include them in any order possible.
+  However, to not clutter global includes, and to make sure internal
+  definitions will not affect global headers, please always include the
+  headers of external components first (these are all headers enclosed
+  in <>), followed by our own exported headers (usually everything
+  that's prefixed by "sd-"), and then followed by internal headers.
+  Furthermore, in all three groups, order all includes alphabetically
+  so duplicate includes can easily be detected.
+
+- To implement an endless loop, use "for (;;)" rather than "while
+  (1)". The latter is a bit ugly anyway, since you probably really
+  meant "while (true)"... To avoid the discussion what the right
+  always-true expression for an infinite while() loop is our
+  recommendation is to simply write it without any such expression by
+  using "for (;;)".
+
+- Never use the "off_t" type, and particularly avoid it in public
+  APIs. It's really weirdly defined, as it usually is 64bit and we
+  don't support it any other way, but it could in theory also be
+  32bit. Which one it is depends on a compiler switch chosen by the
+  compiled program, which hence corrupts APIs using it unless they can
+  also follow the program's choice. Moreover, in systemd we should
+  parse values the same way on all architectures and cannot expose
+  off_t values over D-Bus. To avoid any confusion regarding conversion
+  and ABIs, always use simply uint64_t directly.
+
+- Commit message subject lines should be prefixed with an appropriate
+  component name of some kind. For example "journal: ", "nspawn: " and
+  so on.
+
+- Do not use "Signed-Off-By:" in your commit messages. That's a kernel
+  thing we don't do in the systemd project.
+
+- Avoid leaving long-running child processes around, i.e. fork()s that
+  are not followed quickly by an execv() in the child. Resource
+  management is unclear in this case, and memory CoW will result in
+  unexpected penalties in the parent much much later on.
+
+- Don't block execution for arbitrary amounts of time using usleep()
+  or a similar call, unless you really know what you do. Just "giving
+  something some time", or so is a lazy excuse. Always wait for the
+  proper event, instead of doing time-based poll loops.
+
+- To determine the length of a constant string "foo", don't bother
+  with sizeof("foo")-1, please use STRLEN() instead.
+
+- If you want to concatenate two or more strings, consider using
+  strjoin() rather than asprintf(), as the latter is a lot
+  slower. This matters particularly in inner loops.
+
+- Please avoid using global variables as much as you can. And if you
+  do use them make sure they are static at least, instead of
+  exported. Especially in library-like code it is important to avoid
+  global variables. Why are global variables bad? They usually hinder
+  generic reusability of code (since they break in threaded programs,
+  and usually would require locking there), and as the code using them
+  has side-effects make programs non-transparent. That said, there are
+  many cases where they explicitly make a lot of sense, and are OK to
+  use. For example, the log level and target in log.c is stored in a
+  global variable, and that's OK and probably expected by most. Also
+  in many cases we cache data in global variables. If you add more
+  caches like this, please be careful however, and think about
+  threading. Only use static variables if you are sure that
+  thread-safety doesn't matter in your case. Alternatively consider
+  using TLS, which is pretty easy to use with gcc's "thread_local"
+  concept. It's also OK to store data that is inherently global in
+  global variables, for example data parsed from command lines, see
+  below.
+
+- If you parse a command line, and want to store the parsed parameters
+  in global variables, please consider prefixing their names with
+  "arg_". We have been following this naming rule in most of our
+  tools, and we should continue to do so, as it makes it easy to
+  identify command line parameter variables, and makes it clear why it
+  is OK that they are global variables.
+
+- When exposing public C APIs, be careful what function parameters you make
+  "const". For example, a parameter taking a context object should probably not
+  be "const", even if you are writing an otherwise read-only accessor function
+  for it. The reason is that making it "const" fixates the contract that your
+  call won't alter the object ever, as part of the API. However, that's often
+  quite a promise, given that this even prohibits object-internal caching or
+  lazy initialization of object variables. Moreover it's usually not too useful
+  for client applications. Hence: please be careful and avoid "const" on object
+  parameters, unless you are very sure "const" is appropriate.
+
+- Make sure to enforce limits on every user controllable resource. If the user
+  can allocate resources in your code, your code must enforce some form of
+  limits after which it will refuse operation. It's fine if it is hard-coded (at
+  least initially), but it needs to be there. This is particularly important
+  for objects that unprivileged users may allocate, but also matters for
+  everything else any user may allocated.
+
+- htonl()/ntohl() and htons()/ntohs() are weird. Please use htobe32() and
+  htobe16() instead, it's much more descriptive, and actually says what really
+  is happening, after all htonl() and htons() don't operate on longs and
+  shorts as their name would suggest, but on uint32_t and uint16_t. Also,
+  "network byte order" is just a weird name for "big endian", hence we might
+  want to call it "big endian" right-away.
+
+- You might wonder what kind of common code belongs in src/shared/ and what
+  belongs in src/basic/. The split is like this: anything that uses public APIs
+  we expose (i.e. any of the sd-bus, sd-login, sd-id128, ... APIs) must be
+  located in src/shared/. All stuff that only uses external libraries from
+  other projects (such as glibc's APIs), or APIs from src/basic/ itself should
+  be placed in src/basic/. Conversely, src/libsystemd/ may only use symbols
+  from src/basic, but not from src/shared/. To summarize:
+
+  src/basic/      → may be used by all code in the tree
+                  → may not use any code outside of src/basic/
+
+  src/libsystemd/ → may be used by all code in the tree, except for code in src/basic/
+                  → may not use any code outside of src/basic/, src/libsystemd/
+
+  src/shared/     → may be used by all code in the tree, except for code in src/basic/, src/libsystemd/
+                  → may not use any code outside of src/basic/, src/libsystemd/, src/shared/
+
+- Our focus is on the GNU libc (glibc), not any other libcs. If other libcs are
+  incompatible with glibc it's on them. However, if there are equivalent POSIX
+  and Linux/GNU-specific APIs, we generally prefer the POSIX APIs. If there
+  aren't, we are happy to use GNU or Linux APIs, and expect non-GNU
+  implementations of libc to catch up with glibc.
+
+- Whenever installing a signal handler, make sure to set SA_RESTART for it, so
+  that interrupted system calls are automatically restarted, and we minimize
+  hassles with handling EINTR (in particular as EINTR handling is pretty broken
+  on Linux).
+
+- When applying C-style unescaping as well as specifier expansion on the same
+  string, always apply the C-style unescaping fist, followed by the specifier
+  expansion. When doing the reverse, make sure to escape '%' in specifier-style
+  first (i.e. '%' → '%%'), and then do C-style escaping where necessary.